Recently, in the manufacturing of semiconductor devices, for example, thin-film formation using a plasma CVD (chemical vapor deposition) method has become widely practiced. Plasma CVD has an advantage in that the apparatus having a complex configuration is not necessary and it is possible to perform deposition at relatively low temperature. Moreover, plasma CVD has excellent coverage compared with PVD (physical vapor deposition), such as vacuum deposition and sputtering, and can process a large number of deposition targets at a time depending on the arrangement of the deposition targets; therefore, plasma CVD is used for a variety of purposes.
An explanation will be given of an example of a procedure of plasma CVD that is currently mainstream. A deposition target is placed on a ground electrode in a chamber that is evacuated by a vacuum pump. Reaction gas is introduced into the chamber. High-frequency power is applied from the electrode arranged at a position facing the deposition target, thereby exciting the reaction gas. Deposition is performed on the deposition target due to the chemical reaction between the excited reaction gas and the deposition targets. With this method, a deposition apparatus can have a relatively simple configuration. Moreover, if the size of the facility increases, a deposition process can be performed on a large number of deposition targets. Furthermore, it is not necessary to fix a deposition target; therefore, deposition can also be performed on the entire upper surface of the deposition target.
However, in the case of this method, an increase in the size of the facility is inevitable in order to process a large number of deposition targets. Moreover, when what is called a deposition down method, in which deposition is performed from above in a downward direction, is used, particles generated in the chamber are easily deposited on a deposition target and deposition is easily inhibited due to the particles functioning as a mask. In order to solve this problem, recently, the configuration in which a high-frequency electrode and a ground electrode are arranged vertically has started to become widespread. With this configuration, a large number of deposition targets can be placed in a limited facility and thus the size of the facility can be reduced. Moreover, with this configuration, particles can be prevented from falling onto the deposition target surfaces, enabling the maintenance to be simplified and the utilization of the facility to be improved.
In deposition performed using plasma CVD, the plasma output, the distance from the high-frequency electrode to the deposition target surface, the pressure in the chamber, the flow rate of the reaction gas, and the like are factors that affect the film quality. Among them, the plasma output and the distance from the high-frequency electrode to the deposition target surface are factors that can establish a plurality of conditions in one chamber.
Solar cells are being actively developed that are expected to have significantly improved characteristics due to the formation of a thin film on both sides of the substrate. In any of the widespread conventional CVD apparatuses, one side of a deposition target is set as a deposition target surface. Having a device on both sides of which deposition is to be performed increases the number of manufacturing processes because deposition is performed on one side at a time, thereby making the processing complicated. Furthermore, when a thin film is formed on each of both sides of the substrate such that the thin films have thicknesses different from each other, the manufacturing condition is not shared between the facility for deposition on the front side and the facility for deposition on the back side.